1. Field of the Invention
This invention relates to a process for desalting crude petroleum and to apparatus for carrying out the process.
2. Description of the Prior Art
Crude petroleum delivered to refineries contains impurities comprising water, salts in solution and also solid particles. Prior to any processing of the crude petroleum, which processing starts with distillation at atmospheric pressure, it is necessary to effect an operation known by the term "desalting" so as to eliminate these impurities and thus avoid as far as possible corrosion in the equipment and solid deposits in the refinery units.
The desalting operation comprises adding water to the crude petroleum and then forming an emulsion so that intimate contact occurs between the water and the petroleum.
The salts contained in crude petroleum thus pass into solution into the water. The emulsion is then conveyed into a desalter in which the water and crude petroleum are separated. In order to accelerate this separation a high-voltage electrostatic field can be produced in the desalter so as to cause the droplets of water to coalesce.
It is also possible to add a demulsifying agent to the crude petroleum before it enters the desalter.
The temperature inside the desalter is between 90.degree. and 150.degree. C.
In the description which follows the following terms are used:
"water phase" means the aqueous layer containing in particular salts in solution which are separated in the lower part of the desalter; PA1 "crude phase" means the layer of crude petroleum which is separated in the upper part of the desalter; and PA1 "unstable emulsion" means the emulsion of crude petroleum and water admitted into the desalter. PA1 from 49 to 19% by weight of crude petroleum, PA1 from 50 to 80% by weight of water, PA1 at least 1% by weight of insoluble products. PA1 either by continuous skimming through the desalted crude oil, when changing the charge in the distillation unit and using a ligher crude petroleum, for example, PA1 or by large-scale mechanical entrainment, following accidental mixing of the phases in the desalter. PA1 "first demulsifying agent" means the agent injected into the crude petroleum before it enters the desalter; PA1 "second demulsifying agent" means the agent injected into the desalter, into the stable emulsion, so as to resolve said emulsion. PA1 (A) continuously introducing into a desalter an unstable aqueous emulsion of crude petroleum, said unstable aqueous emulsion optionally containing a first demulsifying agent, PA1 (B) resolving said unstable emulsion in said desalter into a water phase and a crude phase, as herein defined, said resolving step being capable of resulting in the formation of an intermediate phase composed of a stable emulsion located between the water phase and the crude phase, and PA1 (C) continuously drawing off the water phase and crude phase from the desalter, said process further comprising the following steps: PA1 a desalter; PA1 a means for supplying to the desalter an unstable aqueous emulsion of crude petroleum, optionally containing a first demulsifying agent; PA1 a means for drawing off the water phase from the desalter; and PA1 a means for drawing off the crude phase from the desalter; said apparatus further comprising:
During the desalting operation a stable emulsion is formed from time to time at the interface of the "water phase" and of the "crude phase". This emulsion, which in the following description will be designated "stable emulsion", cannot be resolved under the temperature conditions prevailing in a desalter and thus it constitutes a separate phase from the "water phase" and the "crude phase".
The "stable emulsion" may contain:
These insoluble products are constituted by about 50 to 70% by weight of mineral compounds, in particular iron compounds (oxides, sulphide), and 50 to 30% of organic compounds (asphaltenes, carbenes). Asphaltenes and carbenes are bituminous compounds soluble in carbon disulphide; carbenes are insoluble in hot benzene, whereas asphaltenes are soluble therein. According to the temperature, asphaltenes are partially soluble in crude petroleum, whereas carbenes are insoluble therein.
The "stable emulsion" is of the "water in oil" type, the droplets of water being dispersed in the crude petroleum. The insoluble products are concentrated at the periphery and in the interior of the droplets of water.
The stable emulsion may form a layer of quite considerable thickness in the desalter, which is a serious disadvantage since the presence of this thick layer severely interferes with the locating of the level of the water/crude-oil interface in the desalter.
Now this locating is extremely important since it is necessary to know at all times the position of this level so as to verify whether the separation of crude oil and water (i.e. the desalting operation) is being carried out correctly.
The locating of the interface level is effected by means of a float contained in a vertical guide tube immersed in the desalter. When the "stable emulsion" is formed, it penetrates into the tube and obstructs the float, thus hindering the locating of interface level.
It is possible to remove the stable emulsion rapidly from the desalter,
The droplets of water, which are charged with insoluble products, are then entrained by the desalted crude petroleum. The insoluble products are then deposited in the exchangers for preheating the crude petroleum arranged upstream of the furnace of the fractionating column. The exchangers rapidly become clogged and this clogging not only necessitates more frequent maintenance operations, but also increased consumption of the fuel required to heat the crude petroleum in the furnace. Moreover, the quality of desalting is reduced as a result of water entrained by the desalted crude oil.
Until now, means have not been found which make it possible to prevent the formation of a thick layer of stable emulsion in a desalter. In fact, the demulsifying agents normally used in desalters, for example products based on copolymers of propylene and ethylene oxide with low molecular weight, are ineffective in breaking down this type of emulsion.
In an article published in the journal FUEL, volume 53, pages 246 to 252 (October 1974), R. C. LITTLE describes a demulsifying agent, di-2-ethylhexyl sodium sulphosuccinate, which is capable of resolving stable emulsions of fuel oil and water, and the Applicants have established that this agent is equally effective for stable emulsions of crude petroleum and water, which are formed in a desalter, when it is injected into the stable emulsion.
U.S.S.R. Pat. Specification No. 468,946 does in fact disclose the injection of a demulsifying agent at the level of the stable emulsion formed in a decanter, under different temperature conditions to those prevailing in a desalter, but this patent does not disclose means which enable the formation of a stable emulsion to be detected and the nature of the demulsifying agent used is not specified.